Nonstandard Interactions

My first project as a graduate student at NC State was investigating the effects of small interactions between neutrinos and matter that aren’t currently predicted by the Standard Model of particle physics. The Standard Model currently governs our understanding of particle physics. It is one of the best theories physicists have ever created; however, we know its wrong.

Neutrinos are known to violate the Standard Model because of flavor oscillation. Neutrinos can be made as one type, and then spontaneously change types as they move around. This effect has been observed, but the Standard Model can’t predict it. This makes neutrinos excellent particles to study to learn more about physics beyond the Standard Model, and to help make a new theory that is more accurate.

If neutrinos are the perfect probe for new physics, then supernovae are the perfect laboratory for them. Supernovae are incredibly energetic events, with an average supernova explosion releasing as much energy in a few hours as the sun will over its entire lifetime. Most of that energy is carried away by the huge numbers of neutrinos that are made during these events.

My work used current upper limits on the scales of the nonstandard interactions (NSI) between neutrinos and electrons, up quarks, and down quarks, and we started a parameter study, examining the effects of these interactions on the neutrino flavor oscillation. The result of this parameter study was the following plot which is color coded according to different observed behaviors.

NSI Parameter Space Plots
Partitioning of the Parameter Space in the normal (right) and inverted (left) hierarchies. The parameter along the x-axis represents the net effect of coupling to the neutron, and the y-axis shows the scale of non-diagonal NSI terms.

Point A

For small values of both NSI parameters used, we saw two prominent effects. First, even at these small values of the NSI parameters, we find an inner or I-resonance that causes 100% conversion of neutrino flavors at 10s of kilometers. This flavor conversion sets up a reversal of the behavior normally associated with each hierarchy with a bipolar/nutation collective effect observed in the Normal Hierarchy and no such behavior in the Inverted Hierarchy.

Point B

As the values of the NSI parameters increase we observe the probabilities begin to show the effects of a Matter Neutrino Resonance (MNR) first discovered in the context of merger-disk scenarios here at NC State. This type of resonance was not expected to occur without an overabundance of anti-neutrinos; however the I-resonance creates conditions allowing for a cancellation of the matter and neutrino-neutrino interaction terms of the Hamiltonian.

Point C

The location and width of the I-resonance is proportional to the two NSI parameters. As these get larger the resonance moves to larger radii and gets wider. Eventually, this causes the I-resonance to overlap with the onset of an MNR as happens at point C. This has interesting effects that are different between the two hierarchies. In the Normal Hierarchy, the MNR dominates and begins from a point before the I resonance has fully converted neutrinos. In the inverted hierarchy, the onset of the MNR is delayed until the I-resonance fully converts.

Point D

Continuing to increase the NSI parameters will eventually cause the I-resonance and MNR to overlap too much and disrupt the MNR. This returns the behavior of the signal back to the same as at point A with an I resonance at a few 10’s of kilometers and bipolar collective effects after that.

Point E

This represents a region of chaotic effects. Such a large value for the neutron coupling with a small off-diagonal NSI contribution means that the I-resonance, MNR, and bipolar effects are all overlapping without fully disrupting each other. Small changes in the parameters here can have a large effect on the survival probabilities as different resonances become more and less dominant.

Point F

In the soft blue and purple regions that surround point F. At this point the I resonance has moved so far out that it disrupts the normal MSW H-resonance that occurs at thousands of kilometers. This effectively inverts the signals for neutrinos and anti-neutrinos in the two different hierarchies.

Relevant Publications

Charles J Stapleford, Daavid J. Väänänen, James P. Kneller, Gail C. McLaughlin, and Brandon T. Shapiro, Phys. Rev. D 94, 093007 (2016)